Lightguide fiber of the type used to carry optical signals is typically fabricated by heating and drawing a portion of a lightguide preform comprised of a refractive core surrounded by a protective glass cladding. Presently, there are several known processes for fabricating preforms. However, the modified chemical vapor deposition (MCVD) process, which is described in U.S. Pat. No. 4,217,027 issued to J. B. MacChesney et al. on Aug. 12, 1980 and assigned to Bell Laboratories, Inc., has been found most useful because the process enables large scale production of preforms which yield very low loss lightguide fiber.
During the fabrication of preforms by the MCVD process, precursor, reactant-containing gases, such as SiCl.sub.4 and GeCl.sub.4 are passed through a rotating starter tube of silica glass. A torch heats the tube from the outside as the precursor gases pass therethrough, causing deposition of submicron-sized doped glass particles on the inside surface of the tube. The torch is repeatedly moved along the axis of the tube to build up layer upon layer of doped glass. Once a sufficient number of layers has been deposited, the starter tube is then heated to achieve collapse thereof to yield the resultant preform.
Increased demand for lightguide fiber has prompted efforts to increase the productivity of the MCVD process. However, the MCVD process rate is limited by the thickness of the walls of the starter tube. To obtain lightguide fiber having optimal optical and mechanical characteristics, the preform must have a core-to-cladding mass ratio within certain specified limits. Increasing the diameter of the starter tube to obtain a larger preform requires that the walls of the starter tube be made thicker to obtain the desired core-to-cladding mass ratio. However, increasing the thickness of the walls of the tube reduces the rate of heat transfer to the reactant-containing gases, thereby increasing the time required to deposit each layer of doped glass particles. If the walls of the tube are too thick, then insufficient heat transfer may occur, causing the tube to distort and the outside thereof to ablate.
One way in which the productivity of the MCVD process can be increased is to first produce an undercladded preform, having a larger than desired core-to-cladding mass ratio and then inserting the preform into an overcladding glass tube which is then collapsed thereabout. This method is referred to as the rod and tube technique.
In the past, insertion of the preform into the overcladding tube has been accomplished manually which incurs certain disadvantages. If the preform contacts the inside surface of the tube during insertion, then the strength of the resultant drawn fiber will be degraded. Further, radial misalignment between the tube and the undercladded preform may occur during manual insertion, causing the resultant drawn fiber to have an eccentric core which will prevent proper splicing of the fiber to another.
U.S. Pat. No. 3,877,912 issued to Shiraishi et al. on Apr. 15, 1975, discloses a method for producing an optical transmission line whose steps are but a variation of the rod and tube technique discussed above. According to the Shiraishi et al. method, a layer of doped glass particles is deposited on the inside surface of a starter cylinder. Instead of collapsing the starter cylinder to obtain an undercladded preform, a glass rod is inserted into the starter cylinder. The starter cylinder, with the rod coaxially inserted therein, is then collapsed. Insertion of the rod into the starter cylinder is fraught with the same problems associated with the insertion of the undercladded preform into the overcladding tube.
Accordingly, there is a need for a technique for accurately inserting a rod, such as a lightguide preform, into a glass tube.